This invention pertains generally to the field of power conversion and more particularly to a pulse width modulated switching power supply with feedback control.
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Pulse width modulated (PWM) switching power supplies offer both compactness and efficiency in a number of different topologies. Boost and buck PWM switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, however, feedback from the secondary (power output) side of the transformer is required to adjust the pulse width modulation of the power switch. To properly compensate the feedback from the secondary requires extra components and often involves expensive re-design, depending upon the particular application.
Prior art isolated power supplies that used feedback only from the primary side of the transformer did not account for current losses encountered in the load. See, e.g., U.S. Pat. No. 5,982,644, (the ""644 patent), which discloses a pulse-width-modulated boost converter coupled to a high voltage converter, which in turn is coupled to the primary side of a transformer. The modulation of the boost converter is adjusted according to an amplified error signal representing the difference between the boost converter""s output voltage and the voltage from a current sensing circuit sensing the current through the primary winding. This error signal has no way of sensing and accounting for the current losses in the load. Thus, the power supply disclosed in the ""644 patent employs a linear regulator on the secondary side of the transformer to maintain a constant voltage over the load. Although this power supply avoids the use of feedback from the secondary side of the transformer, it introduces the expense and loss associated with installing an additional regulator at the load.
There is a need in the art for an improved power supply that that isolates the input and outputs through a transformer without requiring feedback from the secondary side of the transformer, thereby easing design and reducing the component count.
In accordance with one aspect of the present invention, a power supply comprising a voltage regulator coupled to a DC-to-AC switching power converter is provided. The voltage regulator regulates a DC internal voltage output responsive to an error signal. The DC-to-AC switching power converter (DC/AC converter) includes a transformer having a primary winding coupled to one or more power switches and to the DC internal voltage output. A constant duty cycle control circuit switches the one or more power switches at a fixed rate such that an alternating current flows through the primary winding. Because the alternating current through the primary winding has a constant duty cycle, an output voltage across a load coupled to a secondary winding of the transformer is linearly related to the internal voltage output. An error amplifier outputs a control signal, wherein the control signal is proportional to the difference between a voltage proportional to the current through the secondary winding of the transformer and a reference signal proportional to the maximum voltage losses expected at the secondary winding. A signal proportional to the internal voltage output is combined with the control signal to form the error signal.
In accordance with another aspect of the invention, the voltage regulator may be a boost converter, a buck converter, a buck/boost converter or a linear regulator. Suitable DC/AC converters include a half-bridge, full-bridge, flyback, push-pull, or a full-wave resonant transition converter. In a preferred embodiment, the one or more power switches of the DC/AC converter may comprise a first and a second resonant switch wherein the constant duty cycle control circuit alternatively switches the first resonant switch ON when the second resonant switch is OFF and switches the second resonant switch ON when the first resonant switch is OFF such that the ON and OFF times of each switch are substantially equal, whereby the alternating current flowing through the secondary winding has substantially a 50% duty cycle. When the first resonant switch is ON, a resonant capacitor is coupled to the internal voltage output such that a first half cycle of the alternating current flows through the primary winding in a first direction. Alternatively, when the second resonant switch is ON, the resonant capacitor discharges such that a second half cycle of the alternating current flows in a second direction, opposite to the first direction, through the primary winding. The resonant capacitor may be either in series or in parallel with a leakage inductance of the primary winding, forming a series resonant tank or a parallel resonant tank, respectively.
In accordance with a still further aspect of the invention, a power supply comprising a voltage regulator for regulating an internal voltage output responsive to an error signal coupled to a plurality of DC/AC converters is provided. Each DC/AC converter in the plurality of DC/AC converters includes a transformer having a primary winding coupled to one or more power switches and to the internal output voltage. A control circuit controls the one or more power switches such that an alternating current having a constant duty cycle flows through the primary winding. Because the alternating current through the primary winding has a constant duty cycle, the transformer becomes a linear element. An error amplifier within each DC/AC converter outputs a control signal proportional to the difference between a voltage proportional to the current through a secondary winding of the transformer and a reference voltage proportional to the maximum voltage losses at the load. Each control signal is combined together along with a signal proportional to the internal voltage output to form the error signal. A clock coupled to the plurality of DC/AC converters permits the one or more switches within each of the DC/AC converters to be switched synchronously with each other. An intelligent switch may be coupled between the voltage regulator and the plurality of DC/AC converters wherein a given DC/AC converter is coupled to the internal voltage output through the intelligent switch only when required to support a required voltage across the load.
In accordance with a still further aspect of the present invention, methods of generating DC or AC power are provided. In one embodiment, the method comprises generating an internal voltage output using a voltage regulator wherein the regulation is responsive to an error signal. The internal voltage output is coupled to a primary winding of a transformer such that a first current flows in a first direction through the primary winding during a first period. The internal regulated voltage output is then decoupled from the primary winding such that a second current flows in a second direction opposite the first direction through the primary winding during a second period, a duty cycle thereby formed by the first and second period being maintained the same, period-to-period. The difference between a voltage proportional to the current through the secondary winding and a reference voltage proportional to the maximum expected voltage losses at the load forms a control signal. The control signal and a signal proportional to the internal voltage output are combined to create the error signal.
Other aspects and advantages of the present invention are disclosed by the following description and figures.